Depending on equipment variants, electronic motor vehicle brake systems frequently includes additional functions besides the basic ABS function, such as traction control system (TCS), electronic stability program (ESP), adaptive cruise control (ACC) etc., which in part use subfunctions of the ABS control system.
There are various expensive variants of previously described motor vehicle brake systems on the market; such brake systems that can perform the brake regulation task to be carried out without pressure sensors may be mentioned here in connection with the present invention. Practically all the internal computing variables that are necessary for brake regulation are obtained on the basis of the wheel revolution rate sensors connected to the brake system. Thus especially with the controllers, on which the invention is based, there is no pressure sensor for the upstream pressure applied by the driver, that is the TMC (tandem master cylinder) pressure, as is usual with ESP controllers.
For brake systems without TMC pressure sensors, the upstream pressure (TMC pressure) and the wheel pressure are computed with a model, wherein their difference represents the relevant difference pressure for inlet valve control.
It is thus known from WO 2005/007475 AI to determine the upstream pressure (TMC pressure) by means of the stopping behavior of the ABS return pump operating in the PWM mode by using the generator voltage detected during the switch-off phase as a measure of the upstream pressure.
The wheel pressure can be computed iteratively, starting from a starting value at the point in time of the onset of brake regulation by means of a model, which uses as input variables the modeled TMC pressure, the modeled wheel pressure from a previous computational step (from the last loop) and valve switching times of an inlet and outlet valve of the corresponding wheel.
Up to the point in time of the onset of brake regulation, the associated inlet valves are in the normal, i.e. open, state, so that the pressure in the wheel essentially corresponds to the upstream pressure, i.e. to the pressure in the TMC (tandem master cylinder). Since at the point in time the ABS return pump is still not delivering any voltage information, the starting value for TMC pressure and the wheel pressure can be determined by analysis of the deceleration of the vehicle and the time duration between a brake pedal operation and the onset of brake regulation.
For this, reference is made to DE 10 2006 056 673 A1, which describes a brake regulation method for a motor vehicle brake system, which uses the signal of an upstream pressure sensor during anti-locking regulation at least for the computation of the current wheel pressure, and in the event of a failure of the upstream pressure sensor or in the event of a faulty upstream pressure sensor forms a substitute upstream pressure sensor from a vehicle-specific brake pressure-deceleration characteristic stored in the brake system, wherein a special brake pressure-deceleration characteristic is provided both for the rear axle and also for the front axle.
The known brake regulation method models the upstream pressure such that the value determined from the deceleration of the vehicle is subjected by means of the brake pressure-deceleration characteristic to a safety offset or a safety factor in order to prevent the upstream pressure from being underestimated by the controller, whereby otherwise excessively long valve opening times would arise. Furthermore, it is proposed to increase the pressure from the pressure-deceleration characteristic by a factor or an offset if the brake pedal is depressed very rapidly by the driver (so-called hard braking), since otherwise because of dead times within the deceleration signal generation and within the hydraulics there can be differences between the actual deceleration of the vehicle and the corresponding deceleration signal. The deceleration of the vehicle is computed from the signals of wheel revolution rate sensors.
However, the known method, especially for a high actuation speed of the brake pedal, i.e. for so-called hard pedal operation, does not always lead to a satisfactory result with regard to the model upstream pressure, because the deceleration of the vehicle cannot be computed with sufficient quality at the point in time of the start of brake regulation.
Furthermore, a method for modeling an upstream pressure for a motor vehicle brake system is known from DE 102006022701 A1, with which at the start of a deceleration of the vehicle a counter is started that is incremented with a specified value G and is reset if the speed of the vehicle increases. At the start of regulation the upstream pressure is computed from the counter value by multiplication with a vehicle-specific and axle-specific parameter and is used as a starting value for the brake regulation. The value G used as the gradient of the increase models the gradient of the TMC increase for the upstream pressure and is reduced on reaching specified counter values by means of a reduction factor, which causes flattening of the modeled pressure rise gradient. For brake regulation by means of another model, which is known e.g. from WO97/27090, the starting value for the upstream pressure is used to determine a model wheel pressure for regulation.
The known method in accordance with WO97/27090 for determining the model upstream pressure is based on a computation of the locking pressure level at the start of brake regulation based on the deceleration of the vehicle, which is determined from the wheel speed signals.
The object of the invention is to specify a method mentioned above, with which a model upstream pressure can be determined with high reliability and high robustness against disruptive influences.
This object is achieved by a method with the features described herein.